1. Field of the Invention
This invention relates to a method for producing coke by the thermal cracking of petroleum heavy oils e.g. atmospheric residue, vacuum residue, thermally cracked residue, catalytically cracked residue, solvent deasphalted residue, etc.
2. Description of the Prior Art
Current petroleum coke production employs the so-called delayed coker process, in which raw stock vacuum residue formed in a petroleum refining process is heated at about 450.degree..about.500.degree. C. in a heating furnace and then sent to a coke drum to effect thermal cracking in the drum for a long period of time to produce coke. In this process, various improvements have heretofore been made and research works have been continued so as to obtain cracked oils with as high a yield as possible and to make the crystal of coke remaining in the drum as uniform as possible. Further, research works have been made also from the standpoint of energy saving to cope with the recent increase in the price of crude oil. For example, attempts have been made to increase reaction rate or shorten reaction time by using a catalyst, and to promote the growth of crystals of coke formed. As catalyst serving such purposes, there have heretofore been known Lewis acids such as AlCl.sub.3, FeCl.sub.3, ZnCl.sub.2, NiCl.sub.2, SbCl.sub.3 and BF.sub.3. But these catalysts have drawbacks in difficulties of handling and recovery of the catalyst, resulting in great increase of cost and hence have not been used in practical operation.
It has now been found that, in the coking of petroleum heavy oil by thermal cracking, when a solution of a metal salt of dithio acid such as zinc dialkyldithiocarbamate or zinc dialkyldithiophosphate is dissolved in a solvent of petroleum origin, adjusted to a suitable concentration, and continuously injected by an injection nozzle into the liquid or gas phase part within the coke drum at an extremely low feed rate such as 10.sup.-8 to 10.sup.-3 mol/hour per kg of a stock, the distillation-out of the cracked oil is promoted and the residue in the coke drum is quickly coked to give homogeneous coke having a higher fixed carbon content. The above-mentioned feed rate of the additive is in the most suitable range, but the process is operable even at feed rates outside the range. Thus it has been found that if an additive selected from the above-mentioned various kinds of metal salts of dithio acids, organic phosphates or benzothiazoles is added to the reaction system in a very small amount, a cracked oil can be obtained with a higher yield compared with those of the case where no additive is added, and homogeneous coke having a higher fixed carbon content can be obtained in a short period of time.